Changing the Ambience with quality voltage control – the Preamplifier
One of the biggest advantages of having separate preamplifier and power amplifier stages is that the user can swap between the two either for upgrading or listening purposes. The pro audio world may see this in powered mixers versus the un-powered variety but it is a step invariably anyone interested in sound systems will eventually take.
Of course the so called best preamplifiers are supposedly just attenuators so here I need to tread carefully.
The path without a circuit is best
The shortest distance between two paths is invariably the straight wire so presumably we should ban the pre-amp entirely. Not so quick though, the power amplifier usually needs about 1V at it’s input which does mean we need some sort of voltage amplifier of sorts. The attentuation is fine if one is feeding off a CD Player for instance which has a high enough voltage output sufficient to drive most consumer audio amplifiers. Not if one is using a phono cartridge without preamplifier of course. Attentuation is just that, usually a high quality potentiometer used in a passive setup which means less transistors, capacitors, noisy resistors and a hummin’ power supply.
Phono preamplifiers come in all shapes and sizes but is a necessity to amplify the very low mV values from the humble turntable cartridge, moving coil or moving magnet. The MC output, more popular amongst turntable junkies and audiophiles has an output often measured below 1 mV at full amplitude. This means in the wrong setup we are going to get a lot of noise, poor compensation and a very poor listening experience. Didn’t the supplier warn you that the cartridge was not MM? Normally the cost alone is the “verboten’ element. Then we have the preamplifier or phono-preamplifier often advertised as MC ready but is not really, in fact it’s a bloody mess. High quality preamplifiers with MC input are simply put, rather expensive. But you can make your own then…
The vintage era phono preamplifiers are making their mark as well and it’s absolutely no surprise.
Some consumer preamplifiers, especially high end have Hi-Lo MC and MM preamplifier input stages. These come at a price, like the tube variety.
Our article title, “sneaky world of preamplification” is really another way of looking at preamplification because in it’s simplest configuration would be a straight wire. We pay lots of money for high quality equipment and the one which we should be monitoring is this little voltage amplifier, with or without gain, levels and tone controls. Here we need to become aware of some interesting facts:
Build your own
- Price doesn’t necessarily mean great quality. Read about the $5 preamp from Audio Master Class.
- Building your own preamplifier will always sound better than any other – pseudo acoustic syndrome. But hang on, there’s merit to this. You can chop and change, make modular, run off batteries and even bypass for line use. And of course we have those that roll their tubes, why not the ICs.
- Often the price you pay is for cosmetic appearance – what it looks like. Nothing better than having fancy looking gear in the sound room. I think the ART Digital MPA 2 is a typical example of very good looking gear – and the build and sonic quality is exceptional as well. ART are known for this.
- Looking at the Dynaco ST-70 amplifier which was by no means the best looking amplifier (compared to MacIntosh methinks) in the world but having more than 300 000 avid owners bears testament to David Hafler’s technical know how and experience.
- The low noise instrument preamplifier INA217 [pdf spec sheet](replacement for the SSM2017). This little chip in a ludicrously simple configuration outperformed many costly preamplifiers.
The essential recipe to the success of any circuit is simplicity, ease of design and cost. Your cost will nearly always come down to what the end result should look like.
A vintage catch: I had the option to purchase either the NAD 1020 or Hitachi HCA-6500 in the early 1980s. Although I opted for the NAD which I used solely as a preamp in a DJ mixing console I was put off by how weak the pre-amp input board was – the RCA inputs felt weak and pressed in when exchanging input sources, which was quite often. The Hitachi was a more expensive piece of gear and to my ear was a better piece of equipment sonic wise. You can pick the NADs up for over R2 000.00 on eBay, not a bad return on a preamp which I paid R200.00 for. Both, as you will know are very much in demand, the HCA-6500 is known as a sleeper (hidden away, not to be sold), the NAD a workhorse. Also, the NAD, if you do own one, has the same board as the 3020.
Article photo, the ART MPA II Digital
This is the digital version of the older PRO and it has a remarkably quiet front end and even with entry level microphones the sonic output quality is remarkably like, umm, well, tube like. Remember that this is a tube microphone/instrument preamplifier but it reproduces phono (MM) with the proper RIAA compensation and preamplification front end (before the line in) in a remarkably civilised manner. As this was never intended to be used for this purpose I can vouch that the quality is exceptional. (the RIAA circuit is the one sold by Yebo Electronics – due to be upgraded to the one linked above, the Rod Elliott link).